Two years ago, the United Nations' top health body set 2002 as the deadline for getting rid of remaining stocks amid hopes that the killer disease, officially declared eradicated in 1979 after a massive WHO-spearheaded campaign, would never return.
But the deaths of five people from anthrax in the United States following the September 11 attacks raised the specter of extremist political groups getting access to virulent biological agents such as the smallpox virus.
Once one of the world's most feared diseases, smallpox kills about 30 percent of its victims and leaves others disfigured. It used to kill 3 million to 4 million people per year and left millions more scarred and blind.
There is no effective treatment once somebody falls ill, but administering the vaccine in the days following exposure can prevent the disease from developing.
Its eradication was considered one of world medicine's greatest successes and the international community was anxious to crown the achievement by finally eliminating the remaining stocks of the virus.
The currently available vaccines can be fatal in a small number of cases and cannot be given to people with weakened immune systems, including HIV/AIDS sufferers and transplant recipients.
As expected, the World Health Assembly, the WHO's top decision-taking body, accepted a recommendation by the organization's executive board, itself based on the findings of a special expert committee, that the remaining stocks be retained and that the issue be reviewed not later than 2005.
The executive board decision was announced in January following a meeting in Geneva.
U.S. assistant surgeon general Kenneth Bernard told the assembly that smallpox research was necessary because the "events of Sept. 11 have underscored the extent that terrorists are willing to go to."
"In recent years, experts have come to see smallpox as a No. 1 deadly threat," and the danger of deliberate use was "small but growing," he said. "We regard the potential release of smallpox as a critical national security issue, not only for us but for the entire world."
The only official stocks of variola virus, the smallpox agent, are held in the United States and Russia, but officials say that nobody can be sure that some has not fallen into other hands.
The U.S. stocks are kept at the Centers for Disease Control (CDC) in Atlanta, Georgia, while the Russian stocks are at a center in Siberia.
The assembly said that full research results should be made available to all WHO member nations - a decision made after developing countries said they feared rich nations would keep the results to themselves, WHO officials said.
China previously had called for the destruction of the stocks, claiming their very existence presented an enormous risk to the world. But the Chinese delegation agreed to drop the deadline, provided the research was completed as soon as possible and a new date was set for destruction at a later meeting of the assembly.
Although international teams carry out regular checks of the virus storage facilities to ensure maximum security standards, there have been long-standing fears that samples may not be secure.
Russian officials, however, told the assembly that the stocks in their laboratory were completely secure and dismissed the idea that virus samples could be stolen.